The present invention relates to clock recovery circuits in digital data communication systems, and more particularly to clock recovery circuits for recovering clock signals from burst mode signals in the time division multiple-access (TDMA) satellite communication systems or in the message switching system for cable data communication.
In the TDMA system for satellite communications, a plurality of earth stations transmit intermittent signals (bursts) on a common carrier frequency in mutually different, allocated time slots and share a single satellite-carried repeater on a time-division basis. In the message switching system for cable data communications, on the other hand, messages from a plurality of incoming trunk lines are first stored in buffer memories or the like, and subsequently transmitted on outgoing trunk lines to be connected in a burst mode on a time-division basis. These systems for time-division use of radio or cable lines afford high transmission efficiencies, and moreover contribute to flexible system operation.
However, when clock signals between message bursts, i.e. between earth stations or between messages, are unsynchronized, it is required on the receiving side, for each burst received, to recover a clock signal synchronized with the clock on the transmitting side in order to properly decode the received signals. A clock recovery circuit for this purpose generally has a bandpass filter whose center frequency is the clock frequency from the transmitting side, provided to improve a signal-to-noise (S/N) ratio of the recovered clock. Although stable clock signals can be obtained by sufficiently restricting the bandwidth of the band-pass filter, this would result in a longer rise time of the amplitude response of the band-pass filter in the burst mode and therefore in a deteriorated transmission efficiency. Conversely, widening the band to raise the transmission efficiency would adversely affect the stability of the recovered clock. These competing factors become even more critical with an increase in clock pulse rate.